Comparison of SUS321 and SUS321H: Titanium-Stabilized vs High-Carbon Titanium-Stabilized Austenitic Stainless Steel

SUS321 vs SUS321H: What Changes With Carbon?

SUS321 and SUS321H are titanium-stabilized austenitic stainless steels of the same 18Cr-10Ni-Ti family. The core difference is the carbon content: SUS321 limits carbon to 0.08% maximum, while SUS321H is controlled to 0.04-0.10%. Both grades are stabilized with titanium at a level of at least 4 times the carbon content, with a minimum of 0.70%, so that carbon is bound into TiC carbides and cannot deplete chromium at grain boundaries. The higher carbon of SUS321H produces more TiC, which pins grain boundaries and dramatically improves creep strength at elevated temperatures, making it the grade of choice for long-term stress-bearing service between 600°C and 870°C.

Chemical Composition

The composition ranges below follow JIS G4304 for hot-rolled plates and JIS G4303 for bars; identical chemistry is defined in ASTM A240 for UNS S32100 and S32109.

Element	SUS321 (wt%)	SUS321H (wt%)
Carbon (C)	0.08 max	0.04-0.10
Silicon (Si)	1.00 max	1.00 max
Manganese (Mn)	2.00 max	2.00 max
Phosphorus (P)	0.045 max	0.045 max
Sulfur (S)	0.030 max	0.030 max
Chromium (Cr)	17.0-19.0	17.0-19.0
Nickel (Ni)	9.0-12.0	9.0-12.0
Titanium (Ti)	4 x C min, 0.70 min	4 x C min, 0.70 min
Iron (Fe)	Balance	Balance

Mechanical and High-Temperature Properties

At room temperature the two grades are equivalent in the annealed condition; the difference appears only in long-term high-temperature loading.

Property	SUS321	SUS321H
Tensile Strength min (MPa)	515	515
Yield Strength min (MPa)	205	205
Elongation min (%)	40	40
Hardness max (HB)	201	201
Creep Rupture Strength at 700°C / 1000 h (MPa)	60	85
Continuous Service Range (°C)	-196 to 870	-196 to 870, preferred 600-870 for stress-bearing parts

The 1000-hour creep rupture strength of SUS321H at 700°C is roughly 40% higher than that of SUS321, which is the decisive parameter for superheater and steam-line design lives measured in tens of thousands of hours.

Equivalent Grades

Both grades have recognized equivalents in the main international designation systems.

System	SUS321	SUS321H
JIS	SUS321	SUS321H
UNS (ASTM A240)	S32100	S32109
AISI	321	321H
EN 10088	1.4541 (X6CrNiTi18-10)	1.4941 (X7CrNiTi18-10)

Selecting Between SUS321 and SUS321H

Choose SUS321 for general high-temperature components that do not carry sustained stress: aero-engine exhaust pipes, heat exchanger tubes operating below 870°C, and chemical reaction vessels. Choose SUS321H when the component carries long-term stress at 600-870°C, such as boiler superheater and reheater tubes, high-temperature steam pipelines, and gas turbine auxiliary components. Where heavy sections of SUS321H are welded, a post-weld anneal at 850-900°C with air cooling is required to relieve residual stress and restore creep performance. SUS321H typically costs 10-15% more than SUS321 because of the tighter carbon control and testing requirements.

FAQ

Why does SUS321H have higher high-temperature creep strength?

Its controlled carbon content of 0.04-0.10% forms a larger volume of fine TiC carbides that pin grain boundaries, which resist grain-boundary sliding and plastic deformation under long-term stress at elevated temperature. The result is a 1000-hour creep rupture strength at 700°C of about 85 MPa versus 60 MPa for SUS321.

What post-weld heat treatment does SUS321H require?

For thick sections, a post-weld annealing at 850-900°C followed by air cooling is required. This relieves welding residual stress and restores the high-temperature creep performance of the heat-affected zone. Thin sections below the thickness threshold of the applicable code generally do not require it.

Can SUS321 replace SUS321H in high-temperature stress service?

No. Above 600°C, SUS321 has insufficient creep resistance and will show progressive plastic deformation under sustained load, which can lead to premature failure. SUS321H is the required material for stress-bearing components in the 600-870°C range.

Is there a difference in titanium content between the two grades?

Both are stabilized at 4 x C with a minimum of 0.70% titanium. Because SUS321H has a higher carbon content, its actual titanium content is typically somewhat higher, ensuring that enough TiC forms to stabilize the higher carbon.

How do I choose between SUS321 and SUS321H?

Select SUS321 if the service temperature is at or below 600°C and the component carries no sustained high-temperature stress. Select SUS321H if the service temperature is 600-870°C and the component is under long-term stress, for example superheater tubes or steam lines designed to code creep-life rules.